A. Field of the Invention
Broadly speaking, this invention relates to digital transmission systems. More particularly, in a preferred embodiment, this invention relates to methods and apparatus for locating defective repeaters in digital transmission systems of the type that employ four-phase, differentially-encoded, phase-shift keying.
B. Discussion of the Prior Art
The use of high-speed, digital transmission systems is becoming increasingly widespread. Such systems may be used to transmit both pure binary data and digitally encoded analog signals. The transmission medium employed may comprise optical fiber, coaxial cable, millimeter waveguide or microwave radio, the latter being the most widely employed due to its lower initial cost.
A typical digital microwave system will comprise a terminal station at both ends of the system and a plurality of intermediate repeater stations spaced from 1.5 to 25 miles apart, depending upon the transmitting frequencies employed and the nature of the intervening terrain.
Because of the large number of repeaters employed in a typical microwave route and the fact that most of these repeaters are unattended, it becomes important to be able to quickly determine which of the repeaters has become defective if the receiving terminal determines that it is receiving the data signals with an unacceptably high error rate.
One known way to do this is to devise a unique test pattern and to transmit this pattern for comparison within each repeater with a locally stored version of the unique test pattern. If the digital transmission system is of a type that differentially encodes the digital signal prior to transmission, as most do, this prior art testing procedure requires a differential decoder in each repeater, in addition to a parallel-to-serial converter and some suitable means for storing the unique test pattern. Since a differential decoder is not ordinarily required at an intermediate repeater station, this is obviously an expensive solution to the problem. Known variations of this testing procedure eliminate the need for the parallel-to-serial converter and the memory storage means, but the expensive differential decoder is still necessary.